The present invention relates to a process for preparing a hydrophobically modified alkylene oxide urethane polymer, particularly one that is prepared with bismuth catalyst.
Hydrophobically modified ethylene oxide urethane polymers (HEURs) are rheology modifiers commonly used to thicken coatings formulations. HEURs are typically prepared by reacting together a polyethylene glycol, a polyisocyanate, and a capping agent in the presence of an oganotin catalyst; despite the low concentrations of tin catalyst used to prepare the polymer, environmental pressures are motivating the industry to replace organotin catalysts with more environmentally acceptable catalysts such as salts of bismuth. While environmentally friendly, such catalysts are known to cause undesirably high amounts of precipitation or sedimentation in the final product, as well as undesirable color. Accordingly, it would be desirable to find an environmentally acceptable way to prepare HEURs without substantial formation of precipitation or sedimentation or color during heat aging.